The present invention relates to a space switching network which is, under failure-free conditions, substantially non-blocking, and which has a very low level of blocking under significant failure conditions.
Such networks are used in analogue switching systems for the switching of speech in analogue, non-multiplex form. However, at present a more important use of space networks is for the switching of time division multiplex (TDM) highways, which usually nowadays convey speech in pulse-code modulation (PCM) form. Thus the traffic carried by such a system is relatively heavy, so that it is highly desirable for it to be non-blocking. It is also desirable for the blocking level to be as low as possible under even relatively heavy failure conditions.
British Patent Specification No. 1393645 (International Standard Electric Corporation) to which corresponds U.S. Pat. No. 3,920,914 (Regnier et al) describes a telecommunication exchange in which part at least of the switching network is split into two separate parallel-connected sub-networks, each inlet and each outlet being connected to both sub-networks. When a connection is to be set up between an inlet and an outlet two paths are reserved, one in each of the sub-networks. Then one of the paths thus reserved is used for the connections; if that path fails or otherwise becomes unavailable, the other reserved path is used. As will be seen later, the present invention is an extension of the principle of the invention of the above-identified Patent Specification.